Renewable Energy: Geothermal CONTENTS Introduction Shallow Geothermal Energy – How It Works Shallow Geothermal Applications – Types of Loops Benefits of GHP Systems Deep Geothermal Energy Deep Geothermal Reservoirs Enhanced Geothermal Systems Advantages and Disadvantages of Deep Geothermal Energy Technologies Geothermal Heat Pumps Steam Turbines See Also Introduction Geothermal Energy is one of earth’s renewable energy sources, which include solar, wind, tidal, biofuel, biomass and hydro energy. Energy can be captured from the earth’s natural heat. The earth’s temperature increases as the depth increases from the surface. The earth has many layers, but all geothermal energy applications involve exploiting the crust, the upper most layer. High temperatures in the crust range from 200°C to 700°C. Scientist and engineers have been able to drill and excavate to depths where this heat can be used to generate electrical energy or to use the heat for thermal comfort. Technologies can be used at all different depths and temperatures for a variety of uses. Uses include heating homes and buildings, dehumidifying spaces and generating electrical energy for consumption. As these technologies further develop the use is starting to increase across the world. The attached hyperlink is a video[1] explaining the current uses of geothermal energy and its advantages as a renewable resource. Shallow Geothermal Energy – How It Works While the outdoor temperature changes from very warm to very cold throughout the year in most places, the temperature below the earth’s surface remains fairly constant in the temperature range of 45ºF-75ºF.[2] [3] Geothermal heat pumps (GHP) use this steady temperature by drawing heat out from the earth into the home during the winter, and taking heat out of a home and into the earth in the summer.
During the winter the heat is collected by a fluid-filled series of pipes that run under ground or submersed in a lake. The warmed fluid is carried back into the home and then the GHP system uses an electrically powered compressors and heat exchangers in a vapor compression cycle to concentrate the energy and release it inside the home at a higher temperature. In most systems, duct fans distribute the heat to various rooms. Some GHP models are even available with two-speed compressors and variable fans which provides the user with more heating options, as well as energy savings.23
In the summer, the GHP system acts as a refrigerator. Instead of blowing cold air into the home, it draws heat out of the home and expelled through the ground loop to be absorbed by the earth. Unlike most air condition systems which have outdoor compressors, GHP compressors and all components can be installed inside the home, which prevents damage from the elements. 23 Some GHP systems are also equipped to provide all or at least part of a home’s hot water. The same process of drawing the heat from the earth is used in this process; extra equipment is just needed to transfer the energy to the water to heat it. 3 Shallow Geothermal Applications – Types of Loops Loops for residential GHP systems are installed either horizontally or vertically in the ground or submersed in a pond or lake. Most systems used a closed-loop series of pipes, but there are also open-loop systems where two unconnected pipe lines are used to collect and discharge water. 2
Different Types of Closed Loop GHPs http://www.engineer.gvsu.edu/house/images/additional/heatpump.jpg
Horizontal Closed Loop Much like the vertical closed loop series, the horizontal closed loop system also uses the thermal properties of the earth by circulating water or antifreeze through a closed loop network of sealed and pressurized plastic pipe that is buried in the ground. However, instead of installing the plastic pipe vertically the pipe is laid into horizontal trenches typically 4 to 6 feet deep with a length of 75 to 400 feet per ton of air conditioning. Piping can also be coiled to help increase the amount of piping in a given area; this is called a slinky loop. While these systems require more land area, the cost is much lower than the vertical closed loop system. Pond or Lake Submersion If a residence has a large enough body of water the water submersion of the closed loop series is the most cost efficient GHP system. A water or (more likely) antifreeze filled pipe is run underground from the home to the water and is coiled into circles and placed at least eight feet under the surface of the water to prevent freezing. Open Loop These systems use well or surface body water as the heat exchange fluid instead of fluid filled sealed pipes. Once collected water circulates through the system, the water returns to the ground through a recharge well or surface discharge. Open loop systems can only be used where there is a large enough supply of relatively clean water.
Open Loop GH http://www.capitalwell.ca/Geo_Thermal.htm
Vertical Closed Loop
These GHP systems use the natural thermal properties of the earth in a similar way to the Ground Water Energy systems. The difference is the GHP systems circulate water or antifreeze in their closed vertical network of plastic pipes instead of pumping water out of a well and then back into the ground. These vertical wells are drilled to a depth of 100 to 300 feet per ton of air conditioning. The vertical closed loop series is ideally suited for homes with limited land area. Cost Savings
While the initial installation of GHP systems can range from $15,000-$20,000, the systems save enough energy to pay for themselves in 5-10 years. 3[4] Also, because the components are kept indoors they are much less likely to get damaged. The components have an estimated lifetime of 25 years and the loops have an estimated life of 50 years. 23
Energy Savings & Cleaner Energy
The EPA’s Green Power Partnership includes geothermal energy as a clean, renewable energy source (or "Green Power").[5] One of the biggest benefits of GHP systems is that they use 25%–50% less electricity than conventional heating or cooling systems. According to the EPA, geothermal heat pumps can reduce energy consumption, and thus the emissions from standard energy consumption, up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.2[6] GHP systems also have no air emissions other than what is produced from the electricity that powers the compressor, losses only small amounts of water to evaporation, and has a low risk of ground water contamination (especially with closed loop systems).[7] This lower amount of emissions will help reduce the CO2 output of homes, and possibly halt the temperature increases from global warming.
This clean source of energy is becoming more and more popular among residential homes; especially to those trying to have more green buildings and attain LEED certification.[8] Deep Geothermal Energy
Deep geothermal energy is the energy produced within the earth’s crust between 2 and 10 kilometers down. At these depths, the temperatures can range from 200-700 °C [9] [10] [11] . This heat is absorbed by layers of rock within the crust and the natural fluids which are trapped in the rock’s fractures and pores. Typically, the fluid is liquid and gaseous water with some dissolved salts. In comparison to the amount and distribution of oil and gas deposits, the amount of hot rock and water is substantial2.
Historically, this deep energy has only been utilized when the hot water has found its way to the surface, as in a hot spring or geyser, or is otherwise easily accessible, as in areas on the edge of tectonic plates or areas with high levels of volcanism. For example, it was first used to produce electricity in 1904 in the steam fields found in Larderello, Italy. More modern efforts to harness the energy have clustered in Iceland, New Zealand, Japan, the Philippines, and Indonesia, all areas on tectonic boundaries. However, with recent technological advances it has become feasible to be proactive and drill down directly to where these energy resources lie rather than waiting for them to emerge. As the entire planet is covered by rock and contains mostly water, using this energy as an alternative to fossil fuels has great potential. Indeed, development projects have started or are in the process of starting all over Europe, the United States, and Australia24.
The remainder of this section will give an overview of two methods currently used for accessing deep geothermal energy as well as describing its advantages and disadvantages.
The most common method for using deep geothermal energy involves drilling down to a reservoir of hot water and extracting it. As it is brought up to the surface, the sudden reduction in pressure vaporizes the liquid water to steam which is then used to power a turbine electrical generator in a power plant above.
As expected, this method requires three things: heat from the earth, porous rock, and a preexisting reservoir of hot water. As depicted in the following diagram, the reservoir is formed when surface water makes it way down to the porous rock and becomes trapped by a cap of impermeable rock and heated.
The latter two requirements for this method are limitations on the productivity. Both the rock permeability and rate of water replenishment need to be high enough to ensure long-term economical use and these two characteristic are not able to be found together in all areas.
Image from http://www.geo-energy.org/aboutGE/basics.asp
Enhanced Geothermal Systems
Enhanced Geothermal Systems are also referred to as Engineered Geothermal Systems or Hot Dry Rock Geothermal Energy Systems. The basic mechanics of EDS are identical to the method using preexisting reservoirs: hot water and steam are extracted and used to power turbine electrical generators. It is different, however, in that the porous rock and water are artificially put into the system allowing it to be used on a much broader scale.
As shown in the following diagram, an injection well and a network of fractures are drilled into the dry hot rock. Water is injected into the system and extracted through the production well forming a closed loop heating/cooling system.
The output of EDS systems are expected to cool 10°C every 20-30 years which will recover if the rock is allowed to reheat. However, this is not an insurmountable obstacle given that there are likely plenty of sources of hot dry rock to drill into. One report estimates that the EDS resources in the United States if fully exploited would be “sufficient to provide all the world’s current energy needs for several millennia.”
Image from The Future of Geothermal Energy – Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Report from the Massachusetts Institute of Technology, 2006
Advantages and Disadvantages of DeepGeothermal Energy
The advantages of deep geothermal energy are the same as with any geothermal system: there is no consumption of fossil fuels or greenhouse gas emissions and it is renewable and sustainable in the sense that its source is that of heat from the earth which is not likely to disappear in the foreseeable future. Deep geothermal systems do have several distinct disadvantages, however. First, they are somewhat limited to locations where drilling is feasible and where a water reservoir is found, depending on the method used. Second, drilling may release harmful minerals or gases from underground which would have to be treated and disposed of. [12] Third, drilling and water injection may cause ground subsistence or even earthquakes as occurred in Basel, Switzerland in 2006 only eights days after water was injected into the well. Despite these potential drawbacks, though, deep geothermal energy is a good candidate for a replacement for fossil fuels. Geothermal Technologies
Geothermal energy and geothermal heat is commonly used interchangeably, but the technologies differ in the use of earth’s heat. Geothermal heat pumps or ground source heat pumps use the earth’s heat to either cool or heat a medium and transfer this medium through a heat exchanger, which is used to either heat or cool a space requiring climate control. Geothermal energy or Enhanced Geothermal Systems (EGS) uses the earth’s heat to heat water to either form steam to heat another medium to form a vapor. This vapor is then used to propel a turbine runner and generator to form electrical energy. Heat Pumps A geothermal heat pump simply uses the earth’s ability to transfer heat to a medium, through an evaporator, compressor and condenser. There are a few functional modes[13] of a geothermal heat pump, which include:
Cooling with a water heating function – particular use during the summer when one still desires a warm shower or for washing applications
Passive cooling – particular use in areas that have high humidity, this function does not utilize the heat pump.
Standard heating – particular for thermal comfort during cold temperatures
Domestic hot water heating – particular for facilities that operate with the need for heated water (Dormitory)
Components of a geothermal heat pump system include:
Heat Exchangers – open or closed looped systems, orientated vertically or horizontally either in the ground or in a body of water. Condensers and evaporators are also considered heat exchangers.
Medium – liquid used to transfer heat, can either be a refrigerant such as glycol or water. Water has a superior heat transfer rating but is susceptible to freezing during cold temperatures.
Circulators – pumps that allow the medium to pass through its particular process and overcome pipe and equipment friction loss.
Compressor – device that allows the heat in evaporator to move to the condenser where the thermal heat gain can be passed to a heating fixture such as a hot water tank or radiator.
Expansion Valve – a device that release the cold water from the condenser back to evaporator to be reheated.
Steam Turbines A steam turbine is a electromechanically designed equipment that uses expansion of the heated water (steam) or other medium (vapor) to spin the turbine rotor and subsequently spin a generator in motion to form electrical energy. The use of the steam turbine is used in all the types geothermal energy plants, which include dry steam, flash steam and binary cycle. There are two modes in which a turbine can function. Impulse mode is used with high pressure steam, which passes through a nozzle and pushes against the rotor blades. Reaction mode is used with low pressure, which is directed through vanes or wicket gates and spread evenly across the rotor. The reaction mode allows the steam to be in constant contact with the rotor. The vanes or wicket gates can be rotated to increase the rate of steam to the rotor. As the rotor spins it also spins a generator which has been induced with a magnetic field to generate electricity. The electricity is then transformed as required and flows to the electric grid for consumption.
CONTENTS
Introduction
Shallow Geothermal Energy – How It Works
Shallow Geothermal Applications – Types of Loops
Benefits of GHP Systems
Deep Geothermal Energy
Deep Geothermal Reservoirs
Enhanced Geothermal Systems
Advantages and Disadvantages of Deep Geothermal Energy
Technologies
Geothermal Heat Pumps
Steam Turbines
See Also
Introduction
Geothermal Energy is one of earth’s renewable energy sources, which include solar, wind, tidal, biofuel, biomass and hydro energy. Energy can be captured from the earth’s natural heat. The earth’s temperature increases as the depth increases from the surface. The earth has many layers, but all geothermal energy applications involve exploiting the crust, the upper most layer. High temperatures in the crust range from 200°C to 700°C. Scientist and engineers have been able to drill and excavate to depths where this heat can be used to generate electrical energy or to use the heat for thermal comfort. Technologies can be used at all different depths and temperatures for a variety of uses. Uses include heating homes and buildings, dehumidifying spaces and generating electrical energy for consumption. As these technologies further develop the use is starting to increase across the world. The attached hyperlink is a video[1] explaining the current uses of geothermal energy and its advantages as a renewable resource.
Shallow Geothermal Energy – How It Works
While the outdoor temperature changes from very warm to very cold throughout the year in most places, the temperature below the earth’s surface remains fairly constant in the temperature range of 45ºF-75ºF.[2] [3] Geothermal heat pumps (GHP) use this steady temperature by drawing heat out from the earth into the home during the winter, and taking heat out of a home and into the earth in the summer.
During the winter the heat is collected by a fluid-filled series of pipes that run under ground or submersed in a lake. The warmed fluid is carried back into the home and then the GHP system uses an electrically powered compressors and heat exchangers in a vapor compression cycle to concentrate the energy and release it inside the home at a higher temperature. In most systems, duct fans distribute the heat to various rooms. Some GHP models are even available with two-speed compressors and variable fans which provides the user with more heating options, as well as energy savings.23
In the summer, the GHP system acts as a refrigerator. Instead of blowing cold air into the home, it draws heat out of the home and expelled through the ground loop to be absorbed by the earth. Unlike most air condition systems which have outdoor compressors, GHP compressors and all components can be installed inside the home, which prevents damage from the elements. 23
Some GHP systems are also equipped to provide all or at least part of a home’s hot water. The same process of drawing the heat from the earth is used in this process; extra equipment is just needed to transfer the energy to the water to heat it. 3
Shallow Geothermal Applications – Types of Loops
Loops for residential GHP systems are installed either horizontally or vertically in the ground or submersed in a pond or lake. Most systems used a closed-loop series of pipes, but there are also open-loop systems where two unconnected pipe lines are used to collect and discharge water. 2
Much like the vertical closed loop series, the horizontal closed loop system also uses the thermal properties of the earth by circulating water or antifreeze through a closed loop network of sealed and pressurized plastic pipe that is buried in the ground. However, instead of installing the plastic pipe vertically the pipe is laid into horizontal trenches typically 4 to 6 feet deep with a length of 75 to 400 feet per ton of air conditioning. Piping can also be coiled to help increase the amount of piping in a given area; this is called a slinky loop. While these systems require more land area, the cost is much lower than the vertical closed loop system.
Pond or Lake Submersion
If a residence has a large enough body of water the water submersion of the closed loop series is the most cost efficient GHP system. A water or (more likely) antifreeze filled pipe is run underground from the home to the water and is coiled into circles and placed at least eight feet under the surface of the water to prevent freezing.
Open Loop
These systems use well or surface body water as the heat exchange fluid instead of fluid filled sealed pipes. Once collected water circulates through the system, the water returns to the ground through a recharge well or surface discharge. Open loop systems can only be used where there is a large enough supply of relatively clean water.
Vertical Closed Loop
These GHP systems use the natural thermal properties of the earth in a similar way to the Ground Water Energy systems. The difference is the GHP systems circulate water or antifreeze in their closed vertical network of plastic pipes instead of pumping water out of a well and then back into the ground. These vertical wells are drilled to a depth of 100 to 300 feet per ton of air conditioning. The vertical closed loop series is ideally suited for homes with limited land area.
Cost Savings
While the initial installation of GHP systems can range from $15,000-$20,000, the systems save enough energy to pay for themselves in 5-10 years. 3 [4] Also, because the components are kept indoors they are much less likely to get damaged. The components have an estimated lifetime of 25 years and the loops have an estimated life of 50 years. 23
Energy Savings & Cleaner Energy
The EPA’s Green Power Partnership includes geothermal energy as a clean, renewable energy source (or "Green Power").[5] One of the biggest benefits of GHP systems is that they use 25%–50% less electricity than conventional heating or cooling systems. According to the EPA, geothermal heat pumps can reduce energy consumption, and thus the emissions from standard energy consumption, up to 44% compared to air-source heat pumps and up to 72% compared to electric resistance heating with standard air-conditioning equipment.2 [6] GHP systems also have no air emissions other than what is produced from the electricity that powers the compressor, losses only small amounts of water to evaporation, and has a low risk of ground water contamination (especially with closed loop systems).[7] This lower amount of emissions will help reduce the CO2 output of homes, and possibly halt the temperature increases from global warming.
This clean source of energy is becoming more and more popular among residential homes; especially to those trying to have more green buildings and attain LEED certification.[8]
Deep Geothermal Energy
Deep geothermal energy is the energy produced within the earth’s crust between 2 and 10 kilometers down. At these depths, the temperatures can range from 200-700 °C [9] [10] [11] . This heat is absorbed by layers of rock within the crust and the natural fluids which are trapped in the rock’s fractures and pores. Typically, the fluid is liquid and gaseous water with some dissolved salts. In comparison to the amount and distribution of oil and gas deposits, the amount of hot rock and water is substantial2.
Historically, this deep energy has only been utilized when the hot water has found its way to the surface, as in a hot spring or geyser, or is otherwise easily accessible, as in areas on the edge of tectonic plates or areas with high levels of volcanism. For example, it was first used to produce electricity in 1904 in the steam fields found in Larderello, Italy. More modern efforts to harness the energy have clustered in Iceland, New Zealand, Japan, the Philippines, and Indonesia, all areas on tectonic boundaries. However, with recent technological advances it has become feasible to be proactive and drill down directly to where these energy resources lie rather than waiting for them to emerge. As the entire planet is covered by rock and contains mostly water, using this energy as an alternative to fossil fuels has great potential. Indeed, development projects have started or are in the process of starting all over Europe, the United States, and Australia2 4.
The remainder of this section will give an overview of two methods currently used for accessing deep geothermal energy as well as describing its advantages and disadvantages.
The most common method for using deep geothermal energy involves drilling down to a reservoir of hot water and extracting it. As it is brought up to the surface, the sudden reduction in pressure vaporizes the liquid water to steam which is then used to power a turbine electrical generator in a power plant above.
As expected, this method requires three things: heat from the earth, porous rock, and a preexisting reservoir of hot water. As depicted in the following diagram, the reservoir is formed when surface water makes it way down to the porous rock and becomes trapped by a cap of impermeable rock and heated.
The latter two requirements for this method are limitations on the productivity. Both the rock permeability and rate of water replenishment need to be high enough to ensure long-term economical use and these two characteristic are not able to be found together in all areas.
Enhanced Geothermal Systems
Enhanced Geothermal Systems are also referred to as Engineered Geothermal Systems or Hot Dry Rock Geothermal Energy Systems. The basic mechanics of EDS are identical to the method using preexisting reservoirs: hot water and steam are extracted and used to power turbine electrical generators. It is different, however, in that the porous rock and water are artificially put into the system allowing it to be used on a much broader scale.
As shown in the following diagram, an injection well and a network of fractures are drilled into the dry hot rock. Water is injected into the system and extracted through the production well forming a closed loop heating/cooling system.
The output of EDS systems are expected to cool 10°C every 20-30 years which will recover if the rock is allowed to reheat. However, this is not an insurmountable obstacle given that there are likely plenty of sources of hot dry rock to drill into. One report estimates that the EDS resources in the United States if fully exploited would be “sufficient to provide all the world’s current energy needs for several millennia.”
Advantages and Disadvantages of DeepGeothermal Energy
The advantages of deep geothermal energy are the same as with any geothermal system: there is no consumption of fossil fuels or greenhouse gas emissions and it is renewable and sustainable in the sense that its source is that of heat from the earth which is not likely to disappear in the foreseeable future. Deep geothermal systems do have several distinct disadvantages, however. First, they are somewhat limited to locations where drilling is feasible and where a water reservoir is found, depending on the method used. Second, drilling may release harmful minerals or gases from underground which would have to be treated and disposed of. [12] Third, drilling and water injection may cause ground subsistence or even earthquakes as occurred in Basel, Switzerland in 2006 only eights days after water was injected into the well. Despite these potential drawbacks, though, deep geothermal energy is a good candidate for a replacement for fossil fuels.
Geothermal Technologies
Geothermal energy and geothermal heat is commonly used interchangeably, but the technologies differ in the use of earth’s heat. Geothermal heat pumps or ground source heat pumps use the earth’s heat to either cool or heat a medium and transfer this medium through a heat exchanger, which is used to either heat or cool a space requiring climate control. Geothermal energy or Enhanced Geothermal Systems (EGS) uses the earth’s heat to heat water to either form steam to heat another medium to form a vapor. This vapor is then used to propel a turbine runner and generator to form electrical energy.
Heat Pumps
A geothermal heat pump simply uses the earth’s ability to transfer heat to a medium, through an evaporator, compressor and condenser. There are a few functional modes[13] of a geothermal heat pump, which include:
- Cooling with a water heating function – particular use during the summer when one still desires a warm shower or for washing applications
- Passive cooling – particular use in areas that have high humidity, this function does not utilize the heat pump.
- Standard heating – particular for thermal comfort during cold temperatures
- Domestic hot water heating – particular for facilities that operate with the need for heated water (Dormitory)
Components of a geothermal heat pump system include:Steam Turbines
A steam turbine is a electromechanically designed equipment that uses expansion of the heated water (steam) or other medium (vapor) to spin the turbine rotor and subsequently spin a generator in motion to form electrical energy. The use of the steam turbine is used in all the types geothermal energy plants, which include dry steam, flash steam and binary cycle. There are two modes in which a turbine can function. Impulse mode is used with high pressure steam, which passes through a nozzle and pushes against the rotor blades. Reaction mode is used with low pressure, which is directed through vanes or wicket gates and spread evenly across the rotor. The reaction mode allows the steam to be in constant contact with the rotor. The vanes or wicket gates can be rotated to increase the rate of steam to the rotor. As the rotor spins it also spins a generator which has been induced with a magnetic field to generate electricity. The electricity is then transformed as required and flows to the electric grid for consumption.
Components of a steam turbine include[14]:
The Future of Geothermal Energy – Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Report from the Massachusetts Institute of Technology, 2006 available at http://geothermal.inel.gov (last visited 3/25/2009)
Geothermal Energy Association, http://www.geo-energy.org/aboutGE/basics.asp (last visited 3/25/09)
U.S Department of Energy, "A Consumer's Guide to Energy Efficiency and Renewable Energy - Geothermal Heat Pumps." http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 (last visited 4/1/09)
www.geoexchange.org/geothermal/geoexchange-explained/what-is-geoexchange.html (last visited 4/1/09) , http://www.articlexplosion.com/articledetail.php?artid=119588&catid=265 (last visited 3/20/09)
Advantage and Disadvantages of Geothermal Energy, http://www.greenlivinganswers.com/archives/178 (last visited 3/20/09)
Wikipeda Entry – Description of Steam Turbines http://en.wikipedia.org/wiki/Steam_turbine (last visited 3/25/09)
Toshiba – Manufacturer’s Website http://www.tic.toshiba.com.au/product_brochures_and_reference_lists/110gst.pdf (last visited 3/25/09)
New Energy and Fuel - http://newenergyandfuel.com/http:/newenergyandfuel/com/2007/10/23/the-real-massive-geothermal-generator/ (last visited 3/25/09)
Wikipedia Entry – Heat Exchangers http://en.wikipedia.org/wiki/Heat_exchanger (last visited 3/25/09)
Water Furnace - Manufacturer's Website http://www.waterfurnace.com/ (last visited 3/25/09)
[1] http://geothermal.marin.org/video/vid_page.html
[2] U.S Department of Energy, "A Consumer's Guide to Energy Efficiency and Renewable Energy - Geothermal Heat Pumps." http://apps1.eere.energy.gov/consumer/your_home/space_heating_cooling/index.cfm/mytopic=12640 (April 1, 2009)
[3] Geothermal Heat Pump Consortium, "What is GeoExchange?" http://www.geoexchange.org/geothermal/geoexchange-explained/what-is-geoexchange.html (April I, 2009)
[4] Alexander, Maxwell,"Geothermal Heat Pump: How It Works." This Old House Magazine.http://www.thisoldhouse.com/toh/article/0,,20162296,00.html (April 1, 2009)
[5] Environmental Protection Agency,"Green Power Partnership Brochure 2008." 2008. http://www.epa.gov/greenpower/documents/GPPbrochure08.pdf (April 1, 2009)
[6] Health Goods, "Geothermal Heat Pumps Make Sense For Homeowners." http://www.healthgoods.com/Education/Healthy_Home_Information/Space_Heating_and_Cooling/geothermal_for_homeowners.htm (April 1, 2009)
[7] Environmental Protection Agency,"Non-Hydroelectric Renewable Energy." http://www.epa.gov/cleanenergy/energy-and-you/affect/non-hydro.html#geothermal (April 1, 2009)
[8] McQuay Air Conditioning, "Optimizing Geothermal Heat Pump Systems For Higher Efficiency, Maximum LEED Points and Lower Installed Costs." Engineering System Solutions, April 2005. http://www.mcquay.com/mcquaybiz/literature/lit_systems/EngNews/0405.pdf (April 1, 2009)
[9] The Future of Geothermal Energy – Impact of Enhanced Geothermal Systems (EGS) on the United States in the 21st Century, Report from the Massachusetts Institute of Technology, 2006 available at http://geothermal.inel.gov (last visited 3/25/2009)
[10] Geothermal Energy Association, http://www.geo-energy.org/aboutGE/basics.asp (last visited 3/25/09)
[11] , http://en.wikipedia.org/wiki/Hot_dry_rock_geothermal_energy (last visited 3/20/09)
[12] Nash, James, Advantages and Disadvantages of Geothermal Energy//, http://www.articlexplosion.com/articledetail.php?artid=119588&catid=265 (last visited 3/20/09)
[13] http://www.dimplex.de/animationen/waermepumpe-passiv.php?lang=en
[14] ISO 14224:2006, Petroleum, petrochemical and natural gas industries – Collection and exchange of reliability and maintenance data for equipment